Frequency modulation measurement



Feb. 10, 1942. CROSBY I I 2,272,768

FREQUENCY MODULATION MEASUREMENT Filed June 21, 1940 Fig. I.

CARR/ER FREQUENCY aavsmroe E FREQUENCY MODULATOR 140.111.9545 FREQUENCY 05m 44 rm Inventor.- Hovve ard M. Crosby, b y HisAttorneg.

Patented Feb. 10, 1942 FREQUENCY MODULATION MEASUREMENT Howard M. Crosby, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 21, 1940, Serial No. 341,719

21 Claims.

My invention relates to a method and apparatus for measuring frequency modulated waves.

As is well known, frequency modulation is a term applied to modulation effected by varying the frequency of high frequency carrier waves at a rate andin a degree determined respectively by the frequency and amplitude of modulating waves. The amplitude of the carrier waves is preferably maintained constant, as distinguished from amplitude modulation wherein the frequency of the high frequency carrier waves remains constant and the amplitude thereof is varied in accordance with the modulating waves.

In amplitude modulated radio systems the degree of modulation of a modulated carrier wave is commonly measured by means of an oscilloscope or oscillograph having a pair of coordinate deflecting elements to which the modulated car rier waves and modulating waves are respectively applied, thereby causing a projected ray to be deflected in a particular manner. The pattern traced by the ray on a viewing screen or other sensitive surface is determined by the amplitudes of these waves and gives a direct indication of the degree of modulation, as is well understood in the art. However, this method of measurement is unsuitable for the direct measurement of modulation in a frequency modulation system since the amplitude of the modulated waves is not a function of the modulating potentials.

It is accordingly an object of my invention to provide an improved method and means for measuring the frequency characteristics of frequency modulated waves.

It is another object of my invention to provide a method and means for analyzing and indicating directly the degree of modulation of frequency modulated waves.

A further object of myinvention is to provide a method and apparatus for producing a wave pattern in accordance with frequency modulated waves and altering the configuration of said pattern in accordance with waves of a known frequency to give a direct indication of the degree of modulation.

Still another object of my invention is to provide a method and apparatus utilizing a cathode ray oscilloscope or oscillograph to give a direct indication of the degree of modulation of frequency modulated waves and for calibrating the same to measure modulation indirectly.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of opera- 4 Fig. 1.

.Referring now to Fig. 1, my invention is illustrated in apparatus for monitoring a frequency modulation transmitter. The transmitter may comprise any suitable apparatus known to the art for supplying an antenna II] with high frequency energy, the frequency of which is varied in accordance with a source of modulation potentials, for example, a microphone H The details of the transmitter form no part of my invention and are therefore not represented. Briefly, modulating potentials supplied from the source I! are applied to the frequency modulator I2. By means of the modulator I! the frequency ofthe oscillations generated by the carrier frequency generator I3 is varied over a frequency band at a rate and in a degree determined respectively by the frequency and amplitude of the potentials supplied from the modulation source II. The transmitting apparatus may of course include the usual elements for maintaining the amplitude of the modulated waves constant and for amplifying them prior to transmission.

While the modulation source II has been illustrated diagrammatically as a microphone ll it will of course 'be understood that it may-comprise any suitable source. For example, it may be replaced by an audio frequency oscillator generating a tone for test purposes.

projected from a cathode II, illustrated conventionally' as of the indirectly heated type. The potential from a source It, impressed between an anode l9 and the cathode l'l serves to accelerate through, conductor 21.

the ray I6 toward the screen l5. The device is also provided with the usual ra'y deflecting means,

illustrated as the pairs of electro-static deflection plates 20 and 2|, to which control potentials may be applied. One or both of the ray deflecting means may, of course, be of the magnetic type rather than the electrostatic type, if desired. The device may also include other elements such as means for focusing the ray and controlling the intensity thereof which are not shown since they are non-essential to an understanding of my invention.

While the device M has been indicated as a cathode ray oscilloscope, it will be apparent that it may comprise an-oscillograph for producing a permanent record. The electro-optical type of instrument may also be used, where the frequency of the waves under investigation permits, in which a light ray rather than an .electron ray is utilized to trace a two-dimenquency' choke 25 to one of the deflecting plates 20, the other plate 20 being connected to ground An additional potentiometer 28 connected across a source of constant bias potential, illustrated as a battery 29,

is provided for maintaining a controllable 5 amount of direct current bias on the plates 20 for purposes that will be apparent shortly. One terminal of potentiometer 28 and source 29 is grounded through conductor 30, and the adjustable tap 3| is connected to the lower end of potentiometer 2| "through conductor 32 as shown; Thus, the complete circuit including the plates 20 extends from ground through conductor 30, -an adjustable portion of -bias potentiomeend of winding 33' is' grounded and the other .end is connected to one of the vertical deflecting plates 2| through the conductor 34. The complete circuit includingfthe plates 2| extends from ground, through winding 33', conductor 3 plates 2|, and conductor 21 to ground. If desired, a potentiometer or other suitable means, notshown, may be provided for-adjusting the magnitude of the potentials applied to plates 2| from winding 33.

In accordance with my invention and for reasons to be described presently, an adjustable frequency oscillator 35 is provided. This oscillator may be of any well-known construction and is therefore indicated only diagrammatically. It is designed to generate oscillations of a predetermined known frequency which can be adjusted to any selected value within a band of frequencies. For reasons that will be set forth below this band is made wide enough to include the band of high frequencies which may be present in the frequency modulated transmitter output to antenna I0. Thus the oscillator 35 is indicated conventionally'as having its frequency controlled by the variable capacitor 36' included in the frequency deter mining circuit 36. The movable element of capacitor 36' may carry a suitable pointer 31 movable over a scale 38 calibrated in terms of the oscillator frequency.

The oscillator output may be applied to a selected pair of the deflection plates 20 or 2| through the three-position switch 39. With the switch 39 in the upper position, as shown, the oscillator 35 is disconnected entirely from the plates. With the switch in the midposition, the output of oscillator 35 is applied to the vertical deflecting plates 2| concurrently with the potentials supplied from the winding 33'. When the switch. 39 is in the lower position the output is applied to the horizontal deflecting plates 20 concurrently with the potentials derived from the source The high frequency choke 26 serves to prevent the high frequency oscillations from oscillator 35 from finding a low impedance path to ground through potentiometers 24 and 28 when switch 39 is in the lower position.

Considering now the operation of the apparatus just described, assume first that the switch 39 is in the upper position and that oscillations are being supplied to the antenna |0, these oscillations being frequency modulated in acordance with the modulating potentials supplied by the source II. A pattern 50 is produced on the screen l5 of the cathode ray oscilloscope of the general configuration as best indicated in Fig. 2. It is seen that this pattern is substantially rectangular. The height thereof depends on the amplitude of-the frequency modulated waves applied to the vertical deflecting plates 2|. The width depends upon the amplitude of the potentials applied to the horizontal deflecting plates 20 from the modulation source The height of the pattern is therefore substantially constant and. independent of the degree of modulation, while the Width thereof varies directly with the degree of modulation. the instantaneous frequency'of the output wave isa function of the instantaneous amplitude of horizontal axis of the pattern represents a definite instantaneous frequency of the output wave.

Assuming that the apparatus is in operation as just described to produce the pattern shown in Fig. 2, if the switch 39 is now moved to the mid position, so that a potential is now applied to the vertical deflecting plates 2| from the oscillator 35, the pattern is changed to one similar to that indicated at 50' in Fig. 3. It is seen that the shape .of the pattern is still essentially rectangular exceptfor a pair of diametrically opposite peaks 5|. As the frequency of oscillator 35 is varied, ithas been found that both these peaks move horizontally along the upper and lower edges of the pattern, always remaining vertically aligned. 1

The explanation of this phenomenon is thought to be as follows. If the horizontal sweep potentials derived from the source are applied to the plates 20 .and if the oscillator 35 alone is connected to the vertical plates 2|, a rectangu- Since occur exactly at the center of the pattern as lar pattern similar to that of Fig. 2 is produced. However, the waves at all points along the horizontal axis of this pattern are of the same predetermined frequency of oscillator 35. It will be recalled from the previous discussion of Fig. 2 that each point on the horizontal axis of the rectangular pattern produced by the frequency modulated waves corresponds to a different in-- 'stantaneous frequency. If these two. patterns are now combined, by the simultaneous application of oscillations from winding 33 and oscillator .35 upon the deflecting plates 2| and oscillations from modulation source upon plates 20, the pattern of Fig. 3 results. It has been found that the vertical height-of the pattern, excluding the peaks 5|, is substantially determined by the oscillations of greater amplitude applied to the vertical deflecting plates 2|. Preferably, though not necessarily, the oscillations supplied by oscillator 35 are of much less amplitude than those supplied by the winding 33. Thus, appaieritly the pattern produced by the oscillations v of greater amplitude effectively masks the pattern due'to those of lesser amplitude over most of the resultant pattern. However, at one particular point on the horizontal axis the instantaneous frequency of the oscillations supplied. from the winding 33' will be exactly equal to the frequency of the oscillations supplied from the oscillator 35. Waves of the same frequency may add vectorially, and the magnitude of their resultant depends on the phase angle between them. The value of this phase angle is a matter of fortuitous occurrence in this system and probablyshifts rapidly since the two waves are independefitly generated. At some instants of time the two waves will combine additivelyv to produce a resultant greater than either one, producing the peaks 5| in the pattern. Due to the persistence of vision and the retentivity of the fluorescent screen l5, these peaks appear to exist continuously.

While logically it might be supposed that the A pattern shown in Fig. 3 would be produced only when switch 39 is in the mid position, as a matter of fact it has been observed that it also results when switch 39 is in the lower position, connecting oscillator to the horizontal deflecting plates 20. Although I am not able to explain this effect with certainty, it is believed that electrostatic coupling between the two pairsof de-"' flecting plates 20 and 2| in the cathode ray discharge device I4 is responsilotl e Apparently, due to the structure and charac eristics of this particular type of device, enough energy of the fre-' quency of oscillator 35 is coupled to the vertical deflecting plates 2| under these conditions to produce the effect described- While I have suggested theories which seem to me to provide reasonable explanations for the occurrence of these phenomena, of cou'rsetheir correctness or inc'orrectness is immaterial to the practice of my invention; It has been. found from actual experiments that these well-defined peaks do occur under the conditions specified.

Since, as previously mentioned, the position. of the peaks 5| is indicative of the point at which theinstantaneous frequency of the frequency modulated waves equals that of the oscillator 35, a very convenient means for direct frequency measurement is provided. If the frequency of band is determined. Reversing the connections to the plates 20 of course reverses the pattern.

Once the apparatus'is adjusted for a given set of operating conditions, a scale placed on the screen |5 may be calibrated by means of the oscillator 35 for indirect measurements of fre-. quency modulation. ,Figs. 4, 5 and 6 illustrate various scales which may be employed. In Fig. 4, v

for example, the scale is calibrated in terms'of percent modulation above and below the mean carrier frequeney. 1.00 per cent modulation is arbitrarily defined as a certain definite frequency deviation above or below the mean frequency. In Fig. 5 a scale is shown calibrated directly in terms 'of frequency. As illustrated, the scale is designed for transmission apparatus for which the assigned mean frequency is 42.00 megacycles. If the maximum allowable frequency swing is 60 kilocycles above or below this frequency, the upper and lower ends of the scale will correspond to 42.06 and 41.04 megacycles respectively.

By increasingor decreasing the constant potential bias applied to the horizontal deflecting plates 20 from the potentiometer 28, the

entire pattern may be shifted to one side so that only one edge is visible on the screen, and by increasing the magnitude of the modulation potentials applied to these plates from the'poten tiometer 24, the visible portion of the pattern may be expanded horizontally. By thus shifting and expanding the horizontal scale the position oscillator 35 is adjusted to the mean. carrierj frequency of the waves supplied to antenna '|0"- and ifthe frequencymodulation is symmetrical about thishnean carrier frequency, the peaks 5| of the edge of the pattern may be measuredwith much greater accuracy,"either directly or indirectly'. Thus, for example, Fig. 6 illustrates a scale calibrated in termsof per cent modulation near the upperdimit of frequency swing. From this it is a simple matter to determine whether the maximum modulation'swing is being main tained within allowable limits.

Oscillations supplied from' several oscillators ofknown; frequency m y be impressed simul-H taneously on the defiecti g electrodes to give a plurality of peaks each corresponding to the fre;

quency of one of the oscillators. .Thus, for,ex--

ample, an oscillator of highly constantfrequency .may be maintained accurately. a the mean carrier frequency, as by crystal or resonant line control. This will produce an accurately located peak corresponding to the frequency of the un modulated carrier. Other osc llators may i be a'djusted for the upper and lower limits of the Thus,

In such easeadditional amplificationof the. received signals may be necessary to pro- 1. The method of analyzing electrical oscillations whose frequency is varied over a frequency band at a rate and in a degree determined re spectively by the frequency and amplitude of modulating oscillations which comprises the steps of producing an instantaneous two-dimensional pattern, one dimension of which is determined by the oscillations to be analyzed, and the other dimension of which is determined by the modulating oscillations, independently producing oscillations of a predetermined frequency lying within said 'band and altering the configuration of said pattern in accordance with said last-named oscillations.

2. The method of analyzing electrical oscillations whose frequency is varied at a rate and in a degree determined respectively by the frequency and amplitude of modulating oscillations which comprises the steps of producing an instantaneous electrical pattern in rectangular coordinates, the abscissae of which are proportional to the amplitude of said modulating oscillations and the ordinates of which are propor-' tional to the amplitude of said oscillations to be analyzed, independently producing oscillations of a predetermined known frequency within the range of frequencies of the oscillations to be analyzed, and altering the shape of said pattern in accordance with said predetermined frequency to give an indication of the degree of frequency variations in said oscillations to be analyzed.

3. The method of analyzing oscillations which .are frequency modulated over a frequency band in accordance with modulating potentials comprising the steps of producing a two-dimensional pattern whose dimensions are determined by the oscillations to be analyzed and the modulating potentials respectively, independently producing oscillations of a predetermined frequency within said band and altering the configuration of said pattern in accordance with said last-named oscillations.

4. The method of analyzing the frequency characteristics of electrical waves whose frequency is varied over a'band of frequencies as a function of modulating potentials comprising the steps of directing an electron beam against a sensitive surface, deflecting said beam in one direction in accordance with saidwaves to be analyzed, deflecting said beam in another direcsensitive screen, deflecting said beam along a first axis in accordance with said waves to be analyzed, deflecting said beam along a second, mutually perpendicular axis in accordance with said modulating potentials, whereby said beam is caused to traverse said screen in a pattern of substantially rectangular outline and whereby each point on said pattern along said second axis. corresponds to a definite instantaneous frequency of said waves to be analyzed, and calibrating the positions of said points upon said screen in terms of the corresponding instantaneous frequencies, thereby to determine said frequency variations. 6. The method of analyzing the frequency characteristics of frequency modulated electrical oscillations whose instantaneous frequency is a function of' the instantaneous magnitude of modulating oscillations which comprises the steps of projecting an energy-bearing ray against a screen, said ray being effective to produce an indication thereon, deflecting said ray along a first axis in response to said modulated oscillations, deflecting said ray along a-second axis in response to said modulating oscillations, whereby said ray is caused to trace a two-dimensional pattern on said screen and whereby each point on said pattern along said second axis corresponds to a definite instantaneous frequency of said modulated oscillations, and calibrating the positions of said points upon said screen in terms of the corresponding instantaneous frequencies, thereby to determine said frequency characteristics.

7. The method of analyzing the frequency characteristics of frequency modulated electrical oscillations whose instantaneous frequency is a function of the instantaneous magnitude of modulating oscillations which comprises the teps of generating other oscillations of a predetermiffed frequency within the range of frequencies over' which said frequency modulated oscillations are varied, projecting an energy-bearing ray against a surface, said ray being effective to produce an indication thereon, deflecting said ray tion in accordance with said modulating potentials and concurrently deflecting said beam in said first direction in accordance with'o'scillations of a predeterminedknown frequency lying within said band, whereby said beam Itraverses.

a pattern on said surface whose configuration is a function of said charact ristics.

5. The method of anal zing the frequency characteristics of electrical waves whose amplitude is substantially constant and whose frequency is varied over a band of frequencies as a function of modulating potentials comprising the steps of projecting an electron beam against a along an axis in accordance with said modulating oscillations, and conjointly and simultaneously deflecting said ray alon a mutually perpendicular axis in accordance with said frequency modulated oscillations and said oscillations of predetermined frequency, whereby said ray is caused to traverse said surface in a pattern whose shape is a function of said characteristics.

8.The method of determining the frequency.

characteristics of frequency modulated high frequency waves whose instantaneous frequency is a function of the instantaneous amplitude of lower frequency modulating waves comprising the steps of independently generating waves of a known frequency which \may be adjusted to any frequency within the range of frequencies included by said.frequency modulated waves, developing and projecting a stream of electrons against 'a surface, deflecting said stream in one direction in response to said modulating waves and conjointly and simultaneously deflecting said stream in another mutually perpendicular direction in response both to said frequency modulated waves and to said known frequency waves, whereby said stream is caused to traverse said surface in a pattern whose shap is a function of characteristics.

9. The method of determining the degree of said frequency modulation of frequency modulated high frequency waveswhich extend over a continuous frequency band and whose instantaneous frequency is a function of the instantaneous ampli- 2,272,768 tude of modulating waves comprising the steps modulating oscillations comprising, in combinaof producing waves of a predetermined high fretion, means to project an energy-bearing ray quency lying within said band, developing and again a reen. said ray being effective to P projecting a stream of electrons against a surface, uce an indication on said surface, means to. de-

deflecting said stream in one direction in response 5 flect said my alone a first a S in response o a d to said modulating waves, simultaneously demodulated oscillations, means to deflect said ray fleeting said stream in another mutually perpenl n a Second axis n, r p n o Said modudicular direction in response to said frequency l in Oscillations, ,W y S i ray is caused to, modulated waves, whereby a pattern of substantrace a -twoedilnerisionel pa te n on Said s r tially rectangular outline is traversed by said and whereby each point on Said Pattern alo stream upon said surface whose width in said said second axis corresp ds to a definiteinfirst direction is proportional to the width of said s an e us frequency f d modulated oscillaband, and conjointly and simultaneously further none. d a scale p it adjacent s i r n deflecting said stream in said last named direc and calibrated to indicate the p ns of Said toin in response to said predetermined frequency Points ponsaid'sereen in terms of thecorrewaves, whereby diametrically opposite deforma- P g instantaneous frequencies.

tions are produced in oppositevsides of said pat- The combination. in p at for lyztern whose positions correspond to the position ing frequency modulated e e gy whose inof said predetermined frequency within said band. etanteneeue frequency s a f tion of the ina 10, The cqmbinationi in,apparatus for measstantaneous magnitude of modulating potentials, uring frequency modulated waves whose instanof n adjustable source of waves of known fretaneous frequency is a function of the instanquency, cathoderny discharge i e inclu ing taneo magnitude of dm ting wav of a sensitive screen, means to develop a cathode ray source of waves of known frequency, means to therein and to direct it toward said screen and develop and project an energy-bearing ray a P i f coordinate y deflecting means against a surface, said ray being effective to to mpr Said W v t be n lyzed on one of produce an indication on said surface, a pairof e deflecting m a e Simultaneously to ray deflectingmeans each adapted t be impress said modulating potentials on said other ergized to deflect said ray'in one of .two difdeflecting means, means coniointly o impress ferent directions, means to energize one of said Said known frequency, Waves on one of Said d defiecting means in response. to said frequency fleeting e s. and means to adjust the modulated waves means to energize ,the other quency of Said kn w fr q y W v of said deflecting means response to said 15. The combination. with a source Of modumodulating waves, and means to energize one lotion q n y W ves, a high fr que y s urce of said deflecting means in response to said, of waves of substantially constant amplitude and known frequency waves. meanssfor modulating the frequency of said high 11. Apparatus for analyzing frequency moduquency waves over a freq n y band in a delated electrical oscillations .whose frequency is gree d er ined y e s t n ous amplitude varied over band of frequencies in accordance 0f sald modulation frequency an with modula ing potentialscomprising, in com- 40 justable source of waves of a known frequ n bination, a source ofoscillations of a predeterlying Wi Said band, a cathode y discharge mined known frequency lying within said band, device having y developing means and a pa means t ppoject an energy-bearing ray against of coordinate ray deflecting means, means to a surface, said raybeing effective to produce an energize o f So d y deflecting-means With indicationon said surface, means to deflect said said modulation frequency Waves, means to e ray along an axis in response to said oscillations giZe t other 01'. Sa d y deflecting means with to be analyzed, means to deflect said ray along a said frequency modulated high freque y waves. second axis in response to said m'odulating potenh y d ray is caused to traverse a pattern tials, and mean coniointiy to de ect, said ray of substantially a ular out ine representative along said first axis in response to said oscillaso of s eq y band and ea e i t y to ti n of known frequengy i energize one of said ray deflecting means with I 12. Apparatus for analyzing frequency modu- Said known frequency W v e y a d p lated electrical oscillations whose amplitude is rn is locally deformed to give an ndication of substantially constant and whose frequency is the P o o S kn wn r q ywit n said varied over a band of frequencies as a function e di of modulating potentials comprising ln gombina- 16. The CQmbiDEtiOD, with a source 0f modution, means to project an electron beam against letien r q cy waves, a i h freque cy source a 'sensitive screen, means to deflect said beam of waves of Substantially constant amplitude along a first axis in response to said oscillations v and means r lating the frequen y of Said to be analyzed, means to deflect said beam along h q e y W ves er a'frequency band a a second mutually perpendicular axis in response e rmined by h instantaneous p to said modulating potentials, wherebysaid beam tude of Said modulation frequency W v f an is caused to traverse said screen in a pattern of justable source of Wav s f a wn r q ncy substantially rectangular outline and whereby I lying in Sa d ba cathode y discharge each point on said pattern along said second axis device havi r y developin means and impair of corresponds toadefinite instantaneous frequency coordinate ray deflecting means, means to enerof said waves to be analyzed, and a scale posisiz o of saidray d fl t n m ans with said tinned adjacent said screen and calibrated to dulati n qu n y w a s to en r ize indicate the positions or said points upon said e ot f a d ay flect g means with said .screenin'terms of the corresponding instantaneo fr q ncy modulated h fr uency waves,

V o ifrequencies. 1 whereby said ray is caused to traverse a pattern a 13. Apparatus for analyzing the frequency 0f substantiallyjrec outline-representa- 'characteristicsiof frequency modulated electrical tive of said frequencylconjointly to 1 oscillations whose instantaneous frequency is a energize one of said raydeflecting means with function ofthe instantaneous magnitude of '15 saidknown fr q ency Waveswhereby Said Patof a modulation signal, a source of a carrier signal having its frequency modulated in accordance with said modulation signal, means for modifying the amplitude of said carrier signal at least at one of its modulation frequencies, a cathoderay tube having a luminescent screen adapted 'to be scanned by a beam of electrons, means for deflecting said beam in one direction in accordance with said modified frequency-modulated carrier signal, and means for deflecting said beam in another direction in accordance with said modulation signal to produce a pattern on said screen indicative of the frequency excursionsof said frequency-modulated carrier signal,

18. A modulation indicator comprising a source of a modulating signal, a source of a carrier signal having its frequency modulated in accordance with said modulation signal, means for modifying the amplitude of said carrier signal at certain frequencies, a cathode-ray tube having a luminescent screen adapted to be scanned by a beam of electrons, means for deflecting said beam in one direction in accordance with said modified frequency-modulated carrier signal, means for deflecting said beam in another direction in ac-- cordance with said modulation signal to produce a pattern on said screen indicative of the frequency excursions of said frequency-modulated carrier signal, and indicating means positioned adjacent said pattern calibrated to indicate the carrier-frequency excursion as a functionof the amplitude'of said modulation signal.

19. The method of analyzing the frequency characteristics. of electrical waves whose frequency is varied over a band of frequencies as a function of modulating potentials comprising the steps of directing an electron beam against a sensitive surface, deflecting said beam in one dianalyzed, deflecting said beam in another direction in accordance with said modulating potentials and modifying the deflection of said beam in said first direction at least at one pre-.

determined known frequencylying within said band, whereby said beam traverses a pattern on said surface whose configuration is a function of said characteristics.

20. A modulation indicator comprising a source of a modulation signal, a source of a carrier signal having its frequency modulated in accordance with said modulation signal, a cathoderay tube having a luminescent screen adapted to be scanned by a beam of electrons, means for deflecting said beam in one direction in accordance with said modulation signal, means for deflecting said beam in another direction in accordancewith said frequency-modulated carrier signal, and means for modifying the amplitude of deflection of said beam in said second direction at least at one of the frequencies over which said carrier signal is modulated, whereby a pat-'- tern is produced on said screen indicative of the frequency excursions of said frequency-modulated carrier signal.

21. A modulation indicator-comprising a source of a modulation signal, a source'of a carrier signal having its frequency modulated in accordance with said modulation signal, a cathoderay tube having a luminescent screen adapted to be scanned by a beam of electrons, means for deflecting said beam in one direction in accordance with said modulation signal, means for deflecting said beam in another direction in accordance. with said frequency-modulated carrier signal, means for modifying the amplitude of deflection of said beam in said second direction at least at one of the frequencies over which said carrier signal is modulated, whereby a pattern is produced on said screen indicative of the frequency excursions of said frequency-modulated carrier signal, and indicating means positioned adjacent said pattern calibrated to indicate the carrier-frequency excursion as a function of the amplitude of said modulation signal.

. HOWARD M. CROSBY. 

